Controller for agricultural sprayers

ABSTRACT

A controller for agricultural sprayers utilises a detector (23, 123, 148) to generate red, blue and green colour signals across a field of view. The colour signals are used to generate a `its green` or `not green` output to switch a spray nozzle (13) detection of something deemed to be green. The algorithm which determines if there is something which is `green`, rather than `not green`, looks at the level of green component over the red and blue components in the colour signal and if both are exceeded then the decision is that it is `green`. The level of green over each of red and blue can be compared against preset values to determine the `green`, `not green` output. The level of green can be established by summing pixel by pixel over an area within the field of view under consideration to see if the sum for the area exceeds a set level to decide that the area is `green` and requires spraying.

FIELD OF THE INVENTION

THIS INVENTION relates to agricultural sprays used to spot spray weedsand the like. In particular the invention relates to a controller bywhich the spot sprays are selectively activated on determination of theexistence of a weed.

BACKGROUND ART

AU-B-37775/89 (618377), the Australian national phase of PCT/AU-89/00267(WO-89/12510), The Minister for Agricultural and Rural Affairs of theState of New South Wales, discloses a controller for agriculturalsprayers where sensors measure the irradiance and radiance (orirradiance and reflectance) of a target area in two bands (eg. red andnear infra-red) of the electromagnetic spectrum. The measurements areused to control the spray. Control involves a determination of therelationship between the ratios of the radiance (or reflectance) to theirradiance in each band respectively. The major flaw in this system isthat it does not cope with changing light conditions or partly shadedareas in the viewing area. Further it does not provide a size selectionfunction. The plant or weed size at which the controller acts is notable to be adjusted.

Colour analysis is the basis of a variety of discrimination systemsoperating in a range of circumstances. Examples are seen in U.S. Pat.No. 4,653,014 (Omron) and U.S. Pat. No. 4,797,738 (Tohken). Theseoperate with video signals, operating on components therein to establishthe existence of a target condition. In Omron there is seen a totallydigital system which uses the R/S, G/S, and B/S signals (where S=R+G+Band R, G, and B are the red, green and blue components of the videosignal). This system defines specific colour by analyzing its threesignals with reference to upper and lower limits. In Tohken the signalsY (luminance), R-Y and B-Y are compared each with two limit values andanalysis determines specific colour. Neither of these systems enablesuse with sprays in the field where an area which is predominantly green,a weed or other target plant, is to be found in an area of anothercolour, usually colours such as brown which return a green component ina camera output.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a controller foragricultural sprays, which controller is able to function at normaloperational speeds and under varying light conditions, to efficientlylocate weeds and other target plants in the field. Other objects andadvantages will hereinafter become apparent.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the present invention resides in an agricultural spraycontroller by which detection of plants on a surface being treated iseffected so as to enable the spot application thereto of a spray, said,spray controller comprising:

a spray activation means whereby to action a spray device to effect thespraying of a plant;

a control means for delivering a signal to the spray activation means toeffect spraying on detection of a plant;

a detector generating a colour video signal provided in the controlmeans for viewing an area of the surface to be treated and generating anoutput representative of the field of view; and control circuitry in thecontrol means coupled to the output of the detector, said controlcircuitry analyzing the detector output and generating said controlsignal depending on the detection of a plant;

the control circuitry determining the existence of a plant by examiningthe colour components of the video signal, noting pixels which arepredominantly green, and generating the control signal when the numberof predominantly green pixels in an area of the field of view indicatesthe existence of a green plant.

Evaluation of various plants of interest and their typical backgrounds(soil, rock, stubble, etc) has shown that green foliage has a Greencontent higher than the Red and Blue content. The same also holds truefor the so called colour difference signals, typically denoted as R-Y,B-Y, and G-Y, where Y is luminance. There are some advantages to workingwith the colour difference signals. The first is that by using thedifference signals the effects of ambient light levels can be largelyignored. A second advantage is that CCD cameras with colour differenceoutputs are more likely to be available. In the ensuing discussion wherethe system is described without specific reference to luminance eitherform of signal can be worked with and the alternate form will be readilyimplemented by the person skilled in the art, there being no specialskill required to make the adaptation required to enable use of onerather than the other.

The existence of the green colour of a target weed in the output RGBcolour signal of a camera might be determined by a number of processes.

In one form of the invention the Green component of the RGB signal iscompared separately to both of the Red and Blue components and if itexceeds both then an `its green` decision can be made. In a preferredform of this type of controller a suitable selectable offset (settingthe level by which the level of green is to exceed the level of redand/or blue) can be introduced so as to allow for different degrees ofgreen of the weeds being treated. To determine if any pixel is green ornot green, a simple analog comparison can be made between instantaneousR-Y and G-Y signals and also the instantaneous B-Y and G-Y signals. Ifin both cases the G-Y signal is greater, the pixel can be considered tobe green.

In a preferred form of the invention the green state of a pixel isdetermined by operation of an algorithm wherein a pixel is deemed to begreen when both of G>R and B<a set threshold for the blue componentapplies. This algorithm is preferable to the G>R and G>B algorithm abovewhen the electronics to implement it is likely to be noisy and falsegreen decisions are being returned. This is useful in low lightconditions when present commercially available CCD cameras are in use.In this situation there is a component of noise present on the cameraoutput signals. It has been found better to compare the B-Y signal tofixed reference voltage slightly offset from the signal level for black.This yields much better noise immunity while still providing a validimplementation of the above algorithm, since for a `green` pixel the R-Yand B-Y signals are generally below the black signal level.

The detector can be any camera generating a colour output and typicallyit can be based on use of solid state devices such as charge coupleddevices (CCD). The intensity of light which the device is to work withcan vary considerably in open conditions and performance is enhanced byuse of a hood whose function is to smooth out any marked lightvariation.

The detector and control circuitry which is used in the presentinvention is ideally able to locate weeds against a variety ofbackgrounds such as black basalt soils, red soils, bare ground, stubblecovered ground, rough rocky ground, changing light conditions, etc. Itis found that a solid state detector such as a CCD based detector isbest operated slightly out of focus so as to avoid false triggers whichmay otherwise arise when traversing ground having varyingcharacteristics.

The circuitry which operates on the detector's signal is preferably ableto perform its analysis in a short time so as to better typicalefficient travel times of an agricultural spray. This is more readilyenabled at lower costs by means of analogue circuits for processing thedetector output.

The detector of the invention is used to convert an image of an areawhich is covered by the spray to a signal stream containing data whichis equivalent to a picture frame which, when a solid state device isused typically comprises an array of pixels. The Red (R), Green (G) andBlue (B) components (RGB) of each of the pixels can be operated on toestablish the green state of each pixel. A decision to spray might bebased on the green state of a set of particular adjoining pixels oralternately the total or summed green component of a set length of anumber of successive scan lines can be determined as the basis of thedecision. These operations can be performed using either of digital oranalogue techniques, or a combination thereof. The final green statewhich is calculated, is to determine a result being either a spray on,or a spray off decision.

The implementation of the above might be by way of circuitry providing alargely hardware approach to the problem of when to activate a spray orit might involve operations performed largely within a processor whichis programmed to perform the desired functions.

BRIEF DESCRIPTION OF THE DRAWINGS

To enable the invention to be more fully understood, various preferredembodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic plan view of an agricultural sprayer fitted inaccordance with the present invention;

FIGS. 2A, 2B and 2C are diagrammatic views of how the field of view of asensor unit may be utilised to advantage in the invention;

FIG. 3 is a side view showing a spray nozzle spraying a weed detected bythe sensor unit;

FIG. 4 is the diagram of a circuit which may be used in a controller inaccordance with the present invention; and

FIG. 5 is a circuit diagram showing another form for the circuitry for acontroller in accordance with the invention;

FIGS. 6 to 9 illustrate a decision making process as might beimplemented to determine if a detector output contains a plant to besprayed.

DETAILED DESCRIPTION

The agricultural sprayer 10 is typically comprised of an extended boom,or booms supporting a linear array, or arrays of spray heads therealong,which boom, or booms, is or are trailed by, or mounted on a tractor 11or other like type prime mover. Boom 12 can be fitted with a pluralityof spaced apart, individually operable, spray heads comprising spraynozzles 13, arrayed therealong and ideally at regularly spacedintervals. The spray nozzles 13 can be connected to one or more spraytanks such as spray tank 14 by suitable pipes, lines or conduits 15,either individually or off a manifold. The spray heads may be any ofthose known in the art. A standard valve, as utilised in theagricultural spray field can provide the means whereby a single sprayhead is able to be selectively operated. Valve 16 selectively allows theflow of spray chemicals from piping 15 to the nozzles 13, each nozzle 13being selectively operable by selective activation of its respectivevalve under control of a controller connected thereto typically via aselectively operable activator. This is ideally achieved by electricalmeans with the controller switching sprays on via use of solenoids whichopen selected valves in the supply line, or lines to activate theirrespective spray heads. All of these elements can be chosen from amongsta range of readily available, off the shelf lines which will be selectedaccording to standard criteria known to those in the art.

A plurality of the detectors can be provided on the boom 12 of FIG. 1.They can be arrayed therealong so as to cover the width of groundspanned by the boom. The field of view of a single one of the detectorsmay be such as to cover the ground beneath a number of adjacent spraysso that a detector is not required for each spray head. As seen in FIG.3 a detector, typically a CCD based type detector 17 can be mounted in ahousing, enclosure or hood 18 which is open at its bottom and which isarranged to be passed over the surface 19, on which there may be weedsto be sprayed, as the tractor draws the boom thereover. The surfacebeing treated will typically be a field being prepared for a new crop,the field being either cleared of the last crop or having a stubblethereon. The housing 18 can be an opaque hood which is ideally arrangedso as to stop all direct light falling on the target area and that waycausing deep shadows therein. The hood 18 acts to diffuse light in thetarget area, the light being that which passes under the hood, into thefield of view of the detector 17.

When a CCD type detector 17 passes over bare soil or stubble, the CCDtherein converts the image below into an output comprising a string ofpixels each characterised by respective RGB components. The controllercan then determine the greenness of each pixel by manipulations of itscomponents. The signal which is output by the detector 17 can beexamined to determine if the weed covers an area of greater than apreset size. If the green signal exceeds a preset threshold limit atwhich the spray is to be activated, the valve 16 can be activated toswitch flow to the appropriate spray nozzle 13 to spray the weed 24 (seeFIG. 3). The circuitry interconnecting the detector 17 and the nozzles13 can incorporate a time delay so that the spray nozzle operates for apreset time so that all of a target weed's area is sprayed as the boommoves over it.

One CCD detector can run a number of spray heads, depending on the widthof its viewing area, and generally four is typical. The distance fromthe camera to the ground is the factor which determines this. Forexample, if it is desired to use one camera to run six spray nozzlesthen the camera may be set higher to cover a greater area at the ground(see the comparison shown between FIGS. 2A and 2B). Alternatively it ispossible to use a wider angle lens (comparison shown between FIGS. 2Aand 2C). In reference to FIGS. 2A, 2B and 2C, 20 is the camera head, 21is the viewing angle.

The selection of height of the camera and the lens characteristics willideally be decided depending on what in field conditions the machineincorporating the controller is working with in working with a wheatstubble, an acute angle lens mounted higher will allow it to look moreeffectively down into the stubble whereas in the normal bare fallow, awider angle lens could be used to look out further. The screening effectof stubble is enhanced as the viewing angle decreases and the verticalstalks more effectively hide a small or flat weed not raised to the samedegree above ground level.

The light diffusing hood's dimensions are not at all critical. Thedimensions will be varied to allow it to be fitted to different booms.The hood is constructed and mounted to keep direct light from theviewing area.

If external lighting is to be used to allow night time operation, aneven white light mounted in the light diffusing hood could be used.

Referring now to FIG. 4, the output from the CCD 123 is fed through anRGB decoder 140 and respective Red, Green and Blue digitizers 141-143and then to a frame store 144. In the frame store the RGB components ofthe output of the CCD 123 can be stored in digital form. The informationin the frame store 144 can be passed via RGB processor 145 to a Greendiscriminator 146 which monitors the level of the Green component usingan algorithm such as the one described below in greater detail requiringboth of G>R and G>B to exist in a pixel before it is deemed to be greenwith some consideration of the number of green pixels in an area beforethe decision is made to call the area in the field of view green and aweed. Alternately the algorithm which is operated can be G>R and B<a setvalue its described elsewhere herein. The discriminator 146 can operatea solenoid driver 147 which is operably connected to a valve associatedwith spray nozzle to activate it and spray the detected weed.

A size selection section can be employed. This size selection sectioncan be used to check the number of green pixels in an area of the targetarea and if their number is above a preset threshold, it can activatethe solenoid to control the flow of chemicals to the spray nozzle. Thethreshold could be made adjustable so that it can be varied to allow anoperator to select the size of the plant to be detected.

The horizontal field of view of a detector can be divided into a numberof smaller regions to allow a single detector and processing section tocontrol multiple valves and associated sprays which can be activated bysolenoids under control of the controller.

The digital circuit of FIG. 4 has two areas which add considerably tothe cost and complexity. The first is that having the digitizers at theoutput of the detector means that the amount of data to be stored in theframe store for a frame of video data is high (of the order of 1 Mbyte). The second is that in order to have a reasonable range of colourlevels to process, 6 or 8 bit digitizers are required, which for videoapplications are rare and expensive.

In the embodiment of FIG. 5, the front end processing can be performedusing analog componentry. In this case, only a 1 bit digitizer isrequired since the result of the comparison is either "green" or "notgreen". It should be noted that by using this analog implementation, thememory requirements in the frame store are eliminated and no expensivedigitizers are required. The digital processing requirements aresubstantially reduced and the whole system speeded up.

Where determining the number of adjacent pixels digitally can be complexand expensive. A simpler and cheaper method to operate is one whichcounts the total number of green pixels in the horizontal lines insteadof the number of adjacent green pixels and count adjacent verticallines. FIG. 5 is a schematic illustrating the components of a circuitwhich can be used in the controller wherein an "is it green" algorithmis implemented at the front end. The detector 25 outputs its usual RGBcomponents on respective lines 26, 27 and 28 respectively, connected inpairs to comparators with pair 26 and 27 fed to comparator 29 and 27 and28 fed to comparator 30 which each produce a logic "1" (high) when thegreen component of the detector output is higher. The respectivecomparisons are examined by the AND circuit 31 and if both thecomparators are logic "1" (high) ie, G>R and G>B, then a green signal,logic "1" (high) is passed to the one digitizer 32. The level of Greenover Red and Blue can be made adjustable in the comparator circuits 29and 30 by either enhancing the G signal or retarding the Red and Bluesignals, so as to allow adjustment to take account of weeds withdifferent green characteristics. If the comparator which determines G>Bis disconnected from the green component in the detector output and itscomparison is with a set value then the circuit will work with thealgorithm requiring both of G>R and B<the set value to apply.

From the 1 bit digitizer the circuit feeds counters which may be ideallyset up in a microprocessor under software control to implement thefurther processing of the detector output. The one bit digitizerincrements either counter 33 or 34 depending on which region is beinganalyzed, with a programmable threshold therein, and if the number ofgreen pixels in the line of the region being looked at exceeds thisthreshold then that line is considered green by storing a logic "1" inmemory. Once all the lines in the region are analyzed and resultsstored, then the number of green lines are counted and these also haveto exceed a preset threshold (Number) if a spray signal is to begenerated. By using this two count method the width and height of a weedis determined. This reduces the amount of memory required while stillproviding similar results, at faster speed and as before the thresholdcan still be varied to allow selection of the plant size to be detected.For example, if the horizontal field of view of the camera is dividedinto four regions, the counting of the "green" pixels can be performedbefore any data is placed into the memory resulting in only 4 bits ofdata for each horizontal scan by the camera instead of perhaps 640 bitsof data (80 bytes). This represents a reduction in the amount of data tobe processed of over 90%.

The signal generated by the detector typically includes components forthe three colours,' RGB, with each component characterised by both ofhue and luminance. In the above set out front end algorithm, the RGBcomponents can be the detector's values minus a factor which can be theluminance (Y) of the camera signal so as to work with pure coloursignals. Depending on which camera is chosen, its output may be signalswhich are the equivalent of colour minus intensity. In the working withthe signals R-Y, G-Y and B-Y, the controller is working with the purecolour components. These signal levels are normalised so as to producemore significant ratios at the comparators 29 and 30.

There are circumstances when the G>R together with G>B principle willbreak down.

Extreme intensity variations can adversely affect performance by makinga CCD device for example underexpose or saturate. However, intensityvariations can be smoothed out by use of the above described lightdiffusing hood.

In another circumstance, a specific gold colour has green higher thanred even though it is not greenish. This problem might be overcome byseeing how close to G and R signals are and how close the G and B is.This is because the gold colour has a close G and R and nearly no blue.

In yet another circumstance, the CCD camera views dead (golden coloured)grass and sees the dark area in between the dead leaves with a greenhue. This causes false triggers. As the size of the dark areas aregenerally small, size adjustments could be used to cut them out.However, size adjustment would limit the effectiveness of the sizeselectability by which a minimum size of weed to be treated is set.Also, the size of the dark area varies with changes in brightness duringthe day. One solution of this problem is to vary the focus of the cameraslightly off normal. This smears out these particular dark areas to cutthe number of false triggers and they can be all but eliminated. Theaffect of focus could be reproduced within the electronics but as thisincreases complexity, it is best to work within the camera's focus.Focus is an analogue solution to a problem which might be workeddigitally but at added cost.

In FIG. 5, the circuit can account for when a "green" plant isstraddling the boundary between two regions in the camera's horizontalfield of view. Since it is customary to set up the spraying equipment tohave an overlap region between adjacent spray nozzles, it is logicalthat an overlap region should also exist between adjacent regions in the"green" detection system. This can be performed as seen in FIG. 5, byutilising two independent counters 33 and 34 to count the number of"green" pixels, and control when they start and stop so as to provide anoverlap in the counting regions. This is seen in FIG. 5 wherein separategreen pixel counters 33 and 34 are switched by a counter controller 35and their total is compared with a threshold set by variable threshold36. The counters are synchronised so that counter 33 counts pixels insegment 1 (eg, pixels 0 to 140). Counter 34 counts pixels in segment 2(eg, pixels 120-240). This gives an overlap at pixels 120 to 140 when aweed is straddling this area. Counter 33 then counts segment 3 whilstcounter 34 counts segment 4. This is repeated through the range ofpixels returned by the camera. Control counter 35 counts the range andresets the "green" counters 33 and 34.

As stated above the examination of the detector output to determine theexistence therein of a weed can involve, use of a microprocessor whichperforms the algorithm and establishes the green state of an area. FIGS.6 to 9 show in flow chart form the sequence of operations by which aspray activation signal might be generated. This is illustrated withreference to the G>R and G>B version and area calculation based on ascan line approach.

FIG. 6 shows the main process operating with four regions (associatedeach with one of four spray heads). On start up at 150 the scan lineprocess 151 (described below in greater detail with reference to FIG. 7)is implemented. If the first region of a scan line is deemed to be greenand the previous scan line was green in this region, see 153, thencounter is incremented at 154 otherwise it is cleared at 157 and thesecond region is processed (158) in the same manner. If the scan linecounter for region 1 is incremented at 154 then the count is compared at155 with a threshold and if it exceeds it then a solenoid on flag is setat 156 otherwise processing passes to region two. The forgoingprocessing is pursued through the third (159) and fourth (160) regionstill the full frame is determined to be completed at 161. At this pointturn on and turn off times are set for solenoids whose flags are set andprocessing passes to the solenoid control process at 163 (describedbelow in greater detail with reference to FIG. 9.

The scan line process at 151 of FIG. 6 is seen in greater detail in FIG.7. On starting the scan line process at 164 the region process(described below in greater detail with reference to FIG. 8) isimplemented. If the last region on a scan line is determined to beprocessed at 166 then the scan line process exits to the is it greendecision process at 152 of FIG. 6 otherwise the scan line process loops.The region process at 165 is seen in FIG. 8 wherein on its commencementat 168 the detector output is examined pixel by pixel. On receipt of apixel at 169 the algorithm G>R and G>B is implemented at 170. If bothconditions apply then a green pixel counter is incremented at 171otherwise and the end of region is tested at 172 with processing loopedto continue if the end of region is not reached. When it is processescontinues with the green pixel count compared to a threshold at 173. Ifthe threshold is exceeded then a green region flag is set at 174 andprocessing passes back to the scan line process.

The solenoid control process is seen in greater detail in FIG. 9. Whenthe turn on and turn off times have been set for solenoids whose flagsare set (see FIG. 6) the solenoid control process is run. If a solenoidon state is indicated at 181 the solenoid is energised at 182 and so onthrough the set with this program exited at 183 and processing returningto the main process. At some cycle through the solenoid process asolenoid off state will be reached to signal that it is time tode-energise for any solenoid which is currently on.

As hereinbefore described, the circuitry preferably incorporates a timedelay so that the spray nozzle will operate for a preset time after itactivated. A timer circuit might be associated with the solenoid,holding it on for a preset time so that the activation signal need onlybe a switch on pulse. Alternately the activation signal might be held onfor the requisite time.

Various changes and modifications may be made to the embodimentsdescribed and illustrated without departing from the invention ashereinafter set forth in the claims.

Some of the features of the invention may be summarised as follows.

The invention contemplates a first system for determining whether apixel is to be deemed green, i.e:

to use the three R, G, B, signals from the camera (which are threevoltages, or, if the camera has a digital output, three digital signals)directly in the algorithm, whereby the pixel is deemed "green" if, forthe pixel: G>R and G >B

In another algorithm, the pixel is deemed "green" if, for the pixel: G>Rand B <a predetermined value.

The invention also contemplates an alternative system for determiningwhether a pixel is to be deemed green, i.e:

the R, G, B signals from the camera are not used directly in thealgorithm, but rather the R, G, and B signals are aggregated to producea value for the light intensity (luminance, Y) according to theconventional formula:

    Y=0.30*R+0.59*G+0.11*B

Thus, in the alternative, the algorithm for determining whether thepixel is or is not green is: the pixel is deemed "green" if, for thepixel: G-Y>R-Y and G-Y>B-Y.

The invention also contemplates the inclusion of a means for alleviatingthe effects of overexposure and underexposure of the scanned area.

When the areas of extreme light are infrequent, one solution is toactivate the spray solenoids in these areas by default. The addedsecurity of ensuring that no "green" areas are missed is paid for with aslight increase in chemical usage.

To be able to discern these extreme light levels a signal known as"Luminance" is developed from the Red, Green and Blue signals from thecamera. The signal in given as

    Luminance=(0.3*Red)+(0.59*Green)+(0.11*Blue)

Luminance basically represents the image without any colour information,ie: it is what is viewed on a black and white television or on a colourtelevision if the colour control is turned to its minimum position.

Once the luminance signal has been developed, the signal level can bemonitored for the extremes of either underexposure (dark areas) oroverexposure (saturated light areas).

A: These conditions can then be used to either force the system toregard them as "Green" areas and hence use the same control mechanismsas are already present in the system, or

B: Preferably, brought into separate counter system which allowindependent control of these conditions. This added control allows theoperator to decide whether to conserve chemicals, or to ensure that no"green" areas are left unsprayed at the expense of slightly higherchemical usage.

2. PHYSICAL MEANS

Extremes of both underexposure (dark areas) and overexposure (saturatedlight areas) can be reduced to eliminate default spraying with acorresponding reduction in chemical usage to be fixing a light diffusinghood above the target areas and keeps the target area/signal within thedynamic range of the CCD. The reduced levels of ambient light have noadverse effect as the electronic exposure control compensates to matchthe light.

Patent publication DE-4,132,637 might be considered relevant to theinvention, in that it shows a (non-agricultural) weed spray controller,in which weeds are detected by means of a video signal.

We claim:
 1. Method for the spot-application of a spray to green weedsor other green plants in an agricultural field, characterized by thefollowing procedural steps:viewing an area of ground with a coloursensing means, the colour sensing means being effective to scan the areain pixelated fashion and to issue three signals, in respect of eachpixel in turn of the scanned area, the three signals being dependent,respectively, upon the amount of Red, Green and Blue light reaching thecolour sensing means at that pixel; comparing the Green signal of apixel with the Red signal and the Blue signal of the pixel, according toa predetermined algorithm relating the said three signals in respect ofeach pixel of the scanned area and deeming the pixel to have a "green"status or a "not green" status in accordance with the comparison;assimilating the statuses of the pixels in a patch of the pixels, theextent of the patch being defined in that the pixels making up the patchare linked to the other pixels in the patch in accordance with apredetermined degree of spacial and temporal proximity to each otherwithin the scanned area; comparing the aggregate of statuses of thepixels of the patch with a predetermined value, and of deeming thestatus of the patch to be "green" or "not green" in accordance with thecomparison; and in respect of each of a plurality of spray headsoperating a spray head to produce a pulse of spray over a patch inaccordance with the patch having the status of "green".
 2. Method ofclaim 1, wherein:the method includes the step of computing the luminanceY of the pixel; and the Red, Green, and Blue signals as used in thealgorithm are R-Y, G-Y, and B-Y.
 3. Method of claim 1, wherein thealgorithm is of the form in which the status of the pixel is set to"green" if both (a) the Green signal exceeds the Red signal, and (b) theGreen signal exceeds the Blue signal.
 4. Method of claim 1, wherein thealgorithm is of the form in which the status of the pixel is set to"green" if both (a) the Green signal exceeds the Red signal, and (b) theBlue signal is less than a predetermined value.
 5. An agricultural spraycontroller by which to control agricultural spray apparatus, whichcontroller detects green plants on a surface being treated to enable thespot application thereto of a spray, comprising:a surface viewing meansfor generating a pixelated colour video output including red (R), green(G), and blue (B) (RGB) colour components representing its field ofview; a pixel receiving means for determining the red, green, and bluecolour components of the pixels in the video output; agreen-pixel-determining means for determining whether each pixel is tobe deemed green, depending on a relationship of the colour components ofthe pixel; a green-area-determining means for counting whether thenumber of deemed-green pixels in the video output corresponding to anarea of the field of view exceeds a pre-determined number, thepredetermined number is based on the number of deemed-green pixelsdeemed indicative of the presence of a green plant in the area; anoutput means for delivering a spray activation signal responsive to thegreen-area-determining means when the number of deemed-green pixels inthe area exceeds the predetermined number; a first comparator todetermine if G is greater than R; a second comparator to determine if Bis less than a set value; and a processor to produce an activationsignal by which to activate a spray head when G exceeds R and G exceedsB, when B is below the set value.
 6. An agricultural spray controller asclaimed in claim 5 wherein the first and second comparators output to adigitizer through an AND circuit, the digitizer out-putting a green ornot green state pixel by pixel to the processor.
 7. An agriculturalspray controller by which to control agricultural spray apparatus, whichcontroller detects green plants on a surface being treated to enable thespot application thereto of a spray, comprising:a surface viewing meansfor generating a pixelated colour video output including red (R), green(G), and blue (B) (RGB) colour components representing its field ofview; a pixel receiving means for determining the red, green, and bluecolour components of the pixels in the video output; agreen-pixel-determining means for determining whether each pixel is tobe deemed green, depending on a relationship of the colour components ofthe pixel; a green-area-determining means for counting whether thenumber of deemed-green pixels in the video output corresponding to anarea of the field of view exceeds a pre-determined number, thepredetermined number is based on the number of deemed-green pixelsdeemed indicative of the presence of a green plant in the area; and anoutput means for delivering a spray activation signal responsive to thegreen-area-determining means when the number of deemed-green pixels inthe area exceeds the predetermined number, wherein pixels are countedacross a scan line, segment by segment over the area of the field ofview and if the number of pixels deemed to be green in a line in asegment exceeds a threshold then the segment line is deemed green, thenumber of scan lines in a segment deemed green are counted and if theline count exceeds a threshold then a plant is deemed to exist in thatsegment of the field of view.
 8. An agricultural spray controller bywhich to control agricultural spray apparatus, which controller detectsgreen plants on a surface being treated to enable the spot applicationthereto of a spray, comprising:a surface viewing means for generating apixelated colour video output including red (R), green (G). and blue (B)(RGB) colour components representing its field of view; a pixelreceiving means for determining the red, green, and blue colourcomponents of the pixels in the video output; a green-pixel-determiningmeans for determining whether each pixel is to be deemed green,depending on a relationship of the colour components of the pixel; agreen-area-determining means for counting whether the number ofdeemed-green pixels in the video output corresponding to an area of thefield of view exceeds a pre-determined number, the predetermined numberis based on the number of deemed-green pixels deemed indicative of thepresence of a green plant in the area; and an output means fordelivering a spray activation signal responsive to thegreen-area-determining means when the number of deemed-green pixels inthe area exceeds the predetermined number, wherein the RGB colourcomponents of a pixel are examined and if G>R and G>B then the pixel isdeemed to be green.
 9. An agricultural spray controller by which tocontrol agricultural spray apparatus, which controller detects greenplants on a surface being treated to enable the spot application theretoof a spray, comprising:a surface viewing means for generating apixelated colour video output including red (R), green (G), and blue (B)(RGB) colour components representing its field of view; a pixelreceiving means for determining the red, green, and blue colourcomponents of the pixels in the video output: a green-pixel-determiningmeans for determining whether each pixel is to be deemed green,depending on a relationship of the colour components of the pixel; agreen-area-determining means for counting whether the number ofdeemed-green pixels in the video output corresponding to an area of thefield of view exceeds a pre-determined number, the predetermined numberis based on the number of deemed-green pixels deemed indicative of thepresence of a green plant in the area; and an output means fordelivering a spray activation signal responsive to thegreen-area-determining means when the number of deemed-green pixels inthe area exceeds the predetermined number, wherein the RGB colourcomponents of a pixel are examined and if G>R and B<a set threshold forblue then the pixel is deemed to be green.